Aminomethylenephosphonic acid solutions

ABSTRACT

Aqueous solutions of aminomethylenephosphonic acid selected from diethylenetriaminepenta(methylenephosphonic acid). diethylenetriaminetetra(methylenephosphonic acid), diethylenetriaminetri(methylenephosphonic acid) and mixtures thereof are stabilized against crystallization of said aminomethylenephosphonic acid by including non-oxidizing mineral acid in such solutions in an amount stoichiometrically equivalent to at least 10% hydrochloric acid by weight of the solution.

BACKGROUND OF THE INVENTION

This invention relates to aqueous solutions of aminomethylenephosphonicacid (hereinafter referred to as "AMP acid") selected fromdiethylenetriaminepenta(methylenephosphonic acid),diethylenetriaminetetra(methylenephosphonic acid) anddiethylenetriaminetri(methylenephosphonic acid) (hereinafter designated"D5A", "D4A" and "D3A" respectively), having improved storage stability.The aminomethylenephosphonic acids are useful as sequestrants for metalions.

Aqueous solutions of AMP acid are existing commercial products, andthese usually contain a mixture of D5A, D4A and D3A, of which theprincipal component, providing at least 40%, e.g. from 55% to 85% andusually from 60% to 80%, of the total weight of D5A, D4A and D3A, isD5A. The remainder is normally mostly D3A together with a small amountof D4A. Other phosphonic acids, e.g. hydroxymethylenephosphonic acid,may be present in trace amounts. Typical commercial products alcocontain small amounts of phosphorous acid and hydrochloric acid (HCl),the latter in an amount less than 10% by weight, usually not more than9%, by weight of the solution. (In the present description and claims,all percentages are by weight, i.e., grams per 100 grams of solution.)

Conventionally, the commercial aqueous solutions are described in termsof their concentration of "active phosphonic acid" which is measured bytitrating the phosphonic acid groups in such solutions and convertingthe results, using predetermined constants, to the stoichiometricallyequivalent concentration of D5A. The concentration of AMP acid,expressed in this way as active phosphonic acid, is normally from about90% to about 99% and most commonly from about 94% to about 98% of theactual concentration of AMP acid.

To minimize handling and transportation costs, it is preferred to shipand store the AMP acid in relatively concentrated solutions, whichtypically contain from 40% to 55% of active phosphonic acid. A problemwith such solutions, (although the problem also exists with more dilutesolutions), is a tendency for the AMP acid to crystallize, particularlyif the solutions are subjected to sudden changes in environment within astorage temperature range which would be normally from about -5° C. toabout 40° C., (most usually 5°-35° C.) but which exceptionally may be aslow as about -20° C. or as high as about 50° C.

It might be supposed that solubility of the AMP acid would be suppressedby the common ion effect of the hydrochloric acid in the solution, andthat a reduction in the concentration of the HCl might provide animprovement. Alternatively it might be supposed that an increase in theconcentration of HCl in the solution, or the addition of some othermineral acid to the solution, might aggravate the problem. In fact, thisresult is obtained in the case of other aminopolymethylenephosphonicacids such as, e.g. nitrilotri(methylenephosphonic acid) as the limit ofits solubility in aqueous solutions at ordinary temperatures (e.g. 25°C.) is driven to very low levels by the presence of HCl inconcentrations above 10% of such solutions.

Surprisingly, it has been found that solutions of AMP acid do not showthe behavior which would be expected from such considerations, and thatsolutions containing higher concentrations of HCl than the usualsolutions referred to above, i.e., solutions containing 10% or more ofhydrochloric acid, have improved stabilities. Other mineral acids havebeen found to have a similar effect.

Accordingly, the present invention provides aqueous solutions of AMPacid having improved stability against crystallization of the AMP acid,and a method for stabilizing aqueous solutions of AMP acid against suchcrystallization.

SUMMARY OF THE INVENTION

An aqueous solution of the invention comprises an AMP acid selected fromD5A, D4A, D3A and mixtures thereof, and a non-oxidizing mineral acid,the non-oxidizing mineral acid being present in an amount sufficient toinhibit crystallization of AMP acid from the solution, said sufficientamount being an amount of HCl which is at least 10% by weight of thesolution or an equivalent amount of at least one other non-oxidizingmineral acid or of a mixture of HCl with at least one othernon-oxidizing mineral acid.

This amount of acid is equivalent to about 0.27 gram equivalents ofnon-oxidizing mineral acid per 100 grams of solution, the gramequivalent of an acid being the weight in grams of the acid whichcontains one gram of replaceable hydrogen. Irrespective of what may bethe actual degree of ionization of the mineral acid in a solution of theinvention, the gram equivalent weight of HCl is understood as being thesame as its gram molecular weight, the gram equivalent weight ofsulfuric acid is understood as being half its gram molecular weight, thegram equivalent weight of phosphoric acid is understood as beingone-third its gram molecular weight, and so on.

The method of the invention is a method of stabilizing an aqueoussolution of an AMP acid selected from D5A, D4A, D3A and mixtures thereofagainst crystallization of the AMP acid, which comprises addingnon-oxidizing mineral acid to an aqueous solution of the AMP acid toprovide an amount of non-oxidizing mineral acid in the solutionsufficient to inhibit crystallization of AMP acid therefrom, the saidamount being defined above.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term "non-oxidizing mineral acid" refers to amineral acid which does not significantly oxidize D5A, D4A or D3A in anaqueous solution at 25° C. The most preferred of these is HCl butothers, e.g. sulfuric, phosphoric, phosphorous, or hydrobromic, as wellas mixtures of two or more of such mineral acids may be usedsatisfactorily.

The mineral acid can be added to the solution by any suitable procedure.E.g., when such a mineral acid is present, following the reactionemployed in production of the AMP acid, in an amount insufficient forgood stabilization of an aqueous solution containing the AMP acidproduct, more of that mineral acid and/or other such mineral acids maybe subsequently added to reach the total amount required for suchstabilization.

It is often convenient to add the extra mineral acid in the form of anaqueous solution of the acid, e.g. HCl as a 32% aqueous solution orsulfuric acid as a 50% aqueous solution.

Such a procedure involves the addition of water as well as acid, and forthe production of concentrated AMP acid solutions, the evaporation of atleast an equivalent amount of water from the original AMP acid solutionprior to the addition of the mineral acid solution is a preferredprocedure.

The invention has its principal application in the stabilization ofcommercial solutions of AMP acid in concentrated or diluted form, inwhich, as indicated above, AMP acid is present as mixtures of D5A, D4Aand D3A. In practice, the solutions will usually contain at least about10% of active phosphonic acid, but not more than about 60% and moreusually not more than about 55% of active phosphonic acid. Thetemperatures at which such solutions are stabilized in accordance withthis invention are those mentioned hereinbefore, i.e., from about -20°to about 50° C., more importantly from about -5° to about 40° C., andmost importantly from about 5° to about 35° C.

The minimum and optimum amounts of non-oxidizing mineral acid normallyrequired for good stabilization vary with the particular mineral acid ormixture of mineral acids used, and with the concentration of AMP acid.There may also be some variation from batch to batch of commercial AMPacid due possibly to slight variations in the proportions of D5A, D4Aand D3A in the mixture and in the amounts of impurities present.

The preferred mineral acid in the solutions of the invention is HCl.When this is used as the sole non-oxidizing mineral acid, thestabilization of solutions containing up to about 45% of AMP acidusually requires the presence of at least 14.8%, and preferably at least15% of HCl; the optimum concentration of HCl is usually in the range 15to 20%, and often in the range 15 to 17%.

Variations in the minimum amount of HCl required for stabilization ofdifferent batches of AMP acid of approximately the same activephosphonic acid content are most apparent with solutions containingabout 50%, for example from about 48% to about 51% active phosphonicacid (which typically corresponds to about 50 to 53% of AMP acid). It isusually found that a concentration of at least about 11.5%, e.g. 12-13%HCl is required, and for some batches the necessary minimumconcentration of HCl may be higher, for example 14% or 15%.

At AMP acid concentrations above about 50%, the minimum concentration ofHCl required for stabilization is usually at least about 11.5% and moreusually 12-13%.

For all these solutions, the concentration of HCl in practice willnormally not exceed about 20%, and is preferably not greater than about17%, although higher concentrations are within the scope of theinvention.

When mixtures of sulphuric acid and HCl are employed, stabilizationusually requires the presence of at least 0.40 total gram equivalents ofmineral acid per 100 grams of solution. However, concentrations of mixedacid as low as this figure, or even as low as 0.45 total gramequivalents per 100 grams of solution, are in general effective only inconcentrated solutions containing about 45% or more AMP acid. Solutionsthat are more dilute with respect to AMP acid require a higher minimumcombined HCl/H₂ SO₄ content for their stabilization. At about 30% AMPacid, the minimum is usually about 0.5 total gram equivalents of mineralacid per 100 grams of solution. Thus, for solutions containing about 30%or more of AMP acid, a preferred total gram equivalents of HCl/H₂ SO₄per 100 grams of solution is from about 0.41 to about 0.55. At about 10%of AMP acid, the minimum combined HCl/H₂ SO₄ content required forstabilization is usually about 0.6 total gram equivalent of mineral acidper 100 grams of solution.

The relative proportions of HCl and H₂ SO₄ in a mixture of HCl and H₂SO₄ used to stabilize AMP acid solutions in accordance with theinvention may also be a further factor in determining the minimum totalmineral acid concentration required. There are indications that theminimum will decrease as the proportion of HCl in the mixture (on a gramequivalent basis) increases.

In practice, the mineral acid content of solutions stabilized withmixtures of HCl and H₂ SO₄ will normally not exceed the required minimumby more than about 0.2 gram equivalents of total mineral acid per 100grams of solution, but larger amounts than this, e.g. up to about 0.9gram equivalents per 100 grams of solution, may be present.

Stabilization by the addition of phosphoric or phosphorous acids to asolution of AMP acid containing about 5% or less of HCl requires the useof relatively large amounts of H₃ PO₄ or H₃ PO₃. E.g., as shown by thedata set out in the tables below, the stable solutions are those thatcontain at least about 30% by weight of H₃ PO₄ or at least about 40% byweight of H₃ PO₃.

It will be appreciated that, because of the number of variables in thesystem, to specify minimum and optimum amounts of stabilizing mineralacid for all conceivable combinations of variables is impossible.However, the procedure for assessing solution stability as set out belowis straightforward, and its performance in order to determine theamounts of mineral acid required to achieve the object of the inventionin any particular instance is well within the capability of the personskilled in the art.

As a generality, however, solutions containing at least about 30% ofactive phosphonic acid will usually be stable provided they contain morethan 0.40, e.g. from about 0.41 to about 0.55 gram equivalents ofnon-oxidizing mineral acid, per 100 grams of solution. In someinstances, the minimum concentration of mineral acid required forstability will be towards the upper end of this range. Moreover,solutions containing about 50% or more of active phosphonic acid are insome instances stabilized by the presence of 0.40 gram equivalents ofmineral acid or less, provided they contain at least 10%, preferably atleast 12% of HCl.

In addition to the aforementioned AMP acid and non-oxidizing mineralacids, the compositions claimed herein can include various otherconstituents which do not prevent substantial realization of advantagesof this invention. E.g., such other constituents may include other metalsequestrants, e.g. other polymethylenephosphonic acids such asnitrilotri(methylenephosphonic acid),ethylenediaminetetra(methylenephosphonic acid),hexamethylenediaminetetra(methylenephosphonic acid) andtriethylenetetraminehexa(methylenephosphonic acid).

The invention is illustrated by the following Examples.

EXAMPLES 1-34

Solutions 1-34 containing various concentrations of active phosphonicacid and HCl were prepared by additions of 32% HCl solution or 32% HClsolution and water to samples of a commercially available product whichis an aqueous solution containing 6.86% HCl and 50.4% of activephosphonic acid (corresponding to 52% AMP acid of which about 70% isD5A, 28% is D3A and 2% is D4A.

After thorough mixing, each solution was seeded with crystals of pureD5A and stored at 20° C. A solution is regarded as stabilized inaccordance with the invention if the seeds dissolve to give a clearsolution which remains clear for at least 24 hours after seeding. Theobservations are shown below.

    ______________________________________                                                    Total HCl Content                                                        % Active           Gm.Eqs./                                            Solution                                                                             Phosphonic        100 Gms.                                             No.    Acid Content                                                                             %      Soln.   Observations                                 ______________________________________                                        1      35         15.6   0.43    Seeds dissolved                                                               after 1 minute;                                                               solution remained                                                             clear on storage.                            2      35         13.8   0.38    Solution slightly                                                             turbid.                                      3      35         12.0   0.33    Turbid solution                                                               after seeding                                                                 followed by                                                                   slight precipita-                                                             tion within 3                                                                 hours. Precipi-                                                               tate stuck to the                                                             bottom.                                      4      35         10.2   0.28    Same as Solution                                                              No. 3.                                       5      35         8.4    0.23    Turbid solution                                                               after seeding.                                                                Considerable                                                                  deposits after 2                                                              days storage.                                6      30         18.51  0.51    Seeds dissolved                                                               quickly. Clear                                                                solution.                                    7      30         14.91  0.41    Same as Solution                                                              No. 6.                                       8      30         11.31  0.31    Turbid solution                                                               after seeding.                                                                Precipitation                                                                 on storage.                                  9      30         7.71   0.21    Considerable pre-                                                             cipitation after                                                              2 days storage.                              10     30         5.91   0.16    Same as Solution                                                              No. 9.                                       11     30         4.11   0.11    Same as Solution                                                              No. 9.                                       12     25         21.43  0.59    Seeds dissolved                                                               quickly. Clear                                                                solution after                                                                2 minutes.                                   13     25         17.83  0.49    Same as Solution                                                              No. 12.                                      14     25         16.03  0.44    Same as Solution                                                              No. 12.                                      15     25         14.23  0.39    Turbid solution                                                               after seeding.                                                                Very slight sed-                                                              imentation on                                                                 storage.                                     16     25         10.63  0.29    Precipitation on                                                              storage.                                     17     20         24.34  0.67    Clear solution                                                                after seeding.                                                                Solution re-                                                                  mained stable                                                                 on storage.                                  18     20         20.74  0.57    Clear solution                                                                after seeding.                                                                Stable solution.                             19     20         17.14  0.47    Seeds dissolved                                                               quickly. Clear                                                                solution on                                                                   storage.                                     20     20         15.34  0.42    Same as Solution                                                              No. 19.                                      21     20         13.54  0.37    Cloudy solution                                                               after seeding.                                                                Slight sedimen-                                                               tation after                                                                  3 days.                                      22     20         9.94   0.27    Non-stable solu-                                                              tion. Precipita-                                                              tion after seed-                                                              ing.                                         23     15         27.26  0.75    Stable clear                                                                  solution.                                    24     15         23.7   0.65    Stable clear                                                                  solution.                                    25     15         20.0   0.55    Stable clear                                                                  solution.                                    26     15         16.46  0.45    Stable clear                                                                  solution.                                    27     15         14.6   0.40    Cloudy solution                                                               on mixing. Very                                                               slight sedimenta-                                                             tion.                                        28     15         12.86  0.35    Cloudy solution.                                                              Slight sedimen-                                                               tation.                                      29     10         30.17  0.83    Stable clear                                                                  solution.                                    30     10         26.6   0.73    Stable clear                                                                  solution.                                    31     10         23.0   0.63    Stable clear                                                                  solution.                                    32     0          19.37  0.53    Stable clear                                                                  solution.                                    33     0          15.77  0.43    Stable clear                                                                  solution.                                    34     10         12.17  0.33    Cloudy solution                                                               on mixing. Sed-                                                               imentation on                                                                 storage.                                     ______________________________________                                    

From the data above it can be seen that stable solutions are obtained ifthe total HCl content is greater than 0.40 gram equivalents per 100grams of solution, and that this minimum mineral acid concentration isrequired irrespective of the concentration of active phosphonic acidover the range 10%-35% active phosphonic acid.

EXAMPLES 35-42

To prepare Solutions Nos. 35-42, 98% H₂ SO₄ was added to solutionscontaining various concentrations of active phosphonic acid obtained bydiluting a commercial product containing 50% of active phosphonic acid(corresponding to about 51.7% AMP acid). After thorough mixing, eachsolution was seeded with crystals of pure D5A and stored at 20° C.Observations of the stabilities of the solutions are shown in the table.

    ______________________________________                                                  Mineral Acid Content                                                                             Total                                            Solu-                                                                              % Active                Gm. Eqs./                                        tion Phosphonic %      %     100 Gms.                                         No.  Acid Content                                                                             HCl    H.sub.2 SO.sub.4                                                                    Soln.   Observations                             ______________________________________                                        35   30         4.44   30    0.73    Stable clear                                                                  solution.                                36   30         4.44   20    0.53    Stable clear                                                                  solution                                 37   30         4.44   15    0.43    Cloudy solu-                                                                  tion on mix-                                                                  ing; sedimen-                                                                 tation on                                                                     storage.                                 38   30         4.44   10    0.33    Same as                                                                       Solution 37.                             39   10         1.47   50    1.06    Stable clear                                                                  solution.                                40   10         1.47   30    0.65    Stable clear                                                                  solution.                                41   10         1.47   25    0.55    Slight sed-                                                                   imentation                                                                    on storage.                              42   10         1.47   20    0.45    Cloudy solu-                                                                  tion on mix-                                                                  ing. Precip-                                                                  itation on                                                                    storage.                                 ______________________________________                                    

From these results, it can be concluded that to stabilize solutionscontaining 30% active phosphonic acid and the indicated amount of HCl,the addition of H₂ SO₄ to give a total mineral acid concentration ofabout 0.5 gram equivalents per 100 grams of solution is effective instabilizing the solution. For solutions containing 10% active phosphonicacid, the addition of H₂ SO₄ to give a total mineral acid content ofabout 0.6 gram equivalents of acid per 100 grams of solution iseffective.

EXAMPLES 43 AND 44

Solutions 43 and 44 were prepared similarly to Solutions 35-42, butusing glacial phosphoric acid instead of H₂ SO₄.

    ______________________________________                                                  Mineral Acid Content                                                                             Total                                            Solu-                                                                              % Active                Gm. Eqs./                                        tion Phosphonic %      %     100 Gms.                                         No.  Acid Content                                                                             HCl    H.sub.3 PO.sub.4                                                                    Soln.   Observations                             ______________________________________                                        43   30         4.44   34.4  1.17    Stable clear                                                                  solution.                                44   30         4.44   20.3  0.74    Cloudy solu-                                                                  tion on mix-                                                                  ing.                                     ______________________________________                                    

EXAMPLES 45-48

Solutions 45-48 were prepared similarly to Solutions 35-42, but using H₃PO₃ instead of H₂ SO₄.

    ______________________________________                                                  Mineral Acid Content                                                                             Total                                            Solu-                                                                              % Active                Gm. Eqs./                                        tion Phosphonic %      %     100 Gms.                                         No.  Acid Content                                                                             HCl    H.sub.3 PO.sub.3                                                                    Soln.   Observations                             ______________________________________                                        45   20         3.2    42    1.63    Clear solu-                                                                   tion after                                                                    seeding.                                 46   10         1.6    56    2.09    clear solu-                                                                   tion after                                                                    seeding.                                 47   10         1.6    49    1.84    Clear solu-                                                                   tion after                                                                    seeding.                                 48   10         1.6    35    1.32    Cloudy solu-                                                                  tion on mix-                                                                  ing.                                     ______________________________________                                    

EXAMPLES 49-63

The solutions of these Examples were prepared by concentrating a sampleof the commercially available product used in Examples 1-34 to about 70%active acid content, and adding 32% HCl solution, 50% H₂ SO₄ solution,water or combinations thereof to portions of the concentrated solution.After thorough mixing, each solution was seeded with crystals of pureD5A and stored at 20° C.

    ______________________________________                                                  Mineral Acid Content                                                                             Total                                            Solu-                                                                              % Active                Gm. Eqs./                                        tion Phosphonic %      %     100 Gms.                                         No.  Acid Content                                                                             HCl    H.sub.2 SO.sub.4                                                                    Soln.   Observations                             ______________________________________                                        49   50         6.44   14.79 0.48    Clear                                    50   50         5.44   9.58  0.38    Cloudy                                   51   50         12.5   6.34  0.47    Clear                                    52   50         12.5   2.82  0.40    Clear                                    53   50         15.0   --    0.41    Clear                                    54   50         12.5   --    0.34    Clear                                    55   50         11.0   --    0.30    Cloudy                                   56   50         10.0   --    0.27    Precipitate                              57   52         10     --    0.27    Cloudy                                   58   52         11     --    0.30    Cloudy                                   59   52         12.3   --    0.34    Clear                                    60   55         7.08   11.24 0.42    Clear                                    61   55         12.3   --    0.34    Clear                                    62   57         12.4   --    0.34    Clear                                    63   60         12     --    0.33    Clear                                    ______________________________________                                    

Comparison of Examples 49-62 with Examples 35-42 shows that when usingmixtures of HCl and H₂ SO₄, the more concentrated active phosphonic acidSolutions Nos. 49-52 and 60 require less total HCl plus H₂ SO₄ forstabilization than the more dilute Solutions Nos. 35-42.

We claim:
 1. An aqueous solution comprising an aminomethylenephosphonicacid selected from diethylenetriaminepenta(methylenephosphonic acid),diethylenetriaminetetra(methylenephosphonic acid),diethylenetriaminetri(methylenephosphonic acid) and mixtures thereof anda non-oxidizing mineral acid, characterized in that said mineral acid ispresent in an amount sufficient to inhibit crystallization of saidaminomethylenephosphonic acid from the solution, said sufficient amountcomprising an amount of hydrochloric acid which is at least about 12% byweight of the solution.
 2. A solution according to claim 1 wherein saidaminomethylenephosphonic acid is a mixture ofdiethylenetriaminepenta(methylenephosphonic acid),diethylenetriaminetetra(methylenephosphonic acid) anddiethylenetriaminetri(methylenephosphonic acid) containing from about55% to about 85%, based on the weight of the mixture, ofdiethylenetriaminepenta(methylenephosphonic acid).
 3. A solutionaccording to claim 1, wherein said mineral acid is a mixture ofhydrochloric acid and sulfuric acid.
 4. A solution according to claim 1which contains from about 50% to about 60% of saidaminomethylenephosphonic acid.
 5. A solution according to claim 1wherein said amount is sufficient to inhibit said crystallization withina temperature range from about -5° to about 40° C.
 6. A method forstabilizing an aqueous solution of aminomethylenephosphonic acidselected from diethylenetriaminepenta(methylenephosphonic acid),diethylenetriaminetetra(methylenephosphonic acid),diethylenetriaminetri(methylenephosphonic acid) and mixtures thereofagainst crystallization of said aminomethylenephosphonic acid, whichcomprises adding non-oxidizing mineral acid to the solution until theamount of non-oxidizing mineral acid in the stabilized solution issufficient to inhibit said crystallization, said sufficient amountcomprising an amount of hydrochloric acid which is at least about 12% byweight of the solution.
 7. A method according to claim 6 in which saidaminomethylenephosphonic acid is a mixture ofdiethylenetriaminepenta(methylenephosphonic acid),diethylenetriaminetetra(methylenephosphonic acid) anddiethylenetriaminetri(methylenephosphonic acid) containing from about55% to about 85%, based on the weight of said mixture, ofdiethylenetriaminepenta(methylenephosphonic acid).
 8. A method accordingto claim 6 in which the stabilized solution contains from about 30% toabout 55% of said aminomethylenephosphonic acid and from about 0.41 toabout 0.55 gram equivalents of mineral acid per 100 grams of solution.9. A method according to claim 6 wherein the stabilized solutioncontains from about 50% to about 60% of said aminomethylenephosphonicacid.
 10. A method according to claim 6 wherein said amount issufficient to inhibit said crystallization within a temperature rangefrom about -5° to about 40° C.
 11. A solution according to claim 1 whichcontains at least about 30% of active aminomethylenephosphonic acid. 12.A solution according to claim 11 wherein said amount of hydrochloricacid is more than 0.40 gram equivalents per 100 grams of solution.
 13. Asolution according to claim 11 containing from about 0.41 to about 0.55gram equivalents of said non-oxidizing mineral acid.
 14. A solutionaccording to claim 13 which contains from about 50% to about 60% of saidaminomethylenephosphonic acid.
 15. A method according to claim 6, saidsolution containing at least about 30% of activeaminomethylenephosphonic acid.
 16. A method according to claim 15, saidsufficient amount comprising an amount of hydrochloric acid which is atleast about 14.8% by weight of the solution.